Consumers increasing want to purchase smaller cellular phones. At the same time, consumers also expect their cellular phones to have increased functionality. These two trends may pose problems for cellular phone manufacturers since the demand for increased functionality tends to increase the size of the cellular phones.
Cellular phones may include MP3 players, personal digital assistants (PDAs), WiFi or other network interfaces, cameras, Bluetooth interfaces and/or other devices. Some new cell phones also incorporate FM radio receivers to allow users to receive analog and/or digital FM broadcasts.
Multi-function cellular phones usually require multiple antennas for receiving analog cellular signals, digital cellular signals, Bluetooth signals, WiFi signals and/or other types of wireless signals. If the cellular phone has an FM receiver, another antenna having suitable dimensions is required to allow reception of the FM signals. Because of the compact dimensions of cellular phones, it is not feasible to integrate a suitable FM antenna within the form factor of the cellular phone. Rather, antennas having larger dimensions are typically required and are externally connected to the cellular phone. For example, the antenna may be combined with and/or run adjacent to wires that connect the FM receiver to earphones such as earbuds.
There are multiple ways of implementing an FM receiver. For purposes of illustration, simplified block diagrams of super-heterodyne and direct conversion transmitter and receiver architectures will be discussed, although other architectures may be used. Referring now to FIG. 1A, an exemplary super-heterodyne receiver 14-1 is shown. The receiver 14-1 includes an antenna 19 that is coupled to an optional RF filter 20 and a low noise amplifier 22. An output of the amplifier 22 is coupled to a first input of a mixer 24. A second input of the mixer 24 is connected to an oscillator 25, which provides a reference frequency. The mixer 24 converts radio frequency (RF) signals to intermediate frequency (IF) signals.
An output of the mixer 24 is connected to an optional IF filter 26, which has an output that is coupled to an automatic gain control amplifier (AGCA) 32. An output of the AGCA 32 is coupled to first inputs of mixers 40 and 41. A second input of the mixer 41 is coupled to an oscillator 42, which provides a reference frequency. A second input of the mixer 40 is connected to the oscillator 42 through a −90° phase shifter 43.
The mixers 40 and 41 convert the IF signals to baseband (BB) signals. Outputs of the mixers 40 and 41 are coupled to BB circuits 44-1 and 44-2, respectively. The BB circuits 44-1 and 44-2 may include low pass filters (LPF) 45-1 and 45-2 and gain blocks 46-1 and 46-2, respectively, although other BB circuits may be used. Mixer 40 generates an in-phase (I) signal, which is output to a BB processor 47. The mixer 41 generates a quadrature-phase (Q) signal, which is output to the BB processor 47.
Referring now to FIG. 1B, an exemplary direct conversion receiver 14-2 is shown. The receiver 14-2 includes the antenna 19 that is coupled to the optional RF filter 20 and to the low noise amplifier 22. An output of the low noise amplifier 22 is coupled to first inputs of RF to BB mixers 48 and 50. A second input of the mixer 50 is connected to oscillator 51, which provides a reference frequency. A second input of the mixer 48 is connected to the oscillator 51 through a −90° phase shifter 52. The mixer 48 outputs the I-signal to the BB circuit 44-1, which may include the LPF 45-1 and the gain block 46-1. An output of the BB circuit 44-1 is input to the BB processor 47. Similarly, the mixer 50 outputs the Q signal to the BB circuit 44-2, which may include the LPF 45-2 and the gain block 46-2. An output of the BB circuit 44-2 is output to the BB processor 47.